There is a need for a permanent magnet to have large magnetic flux density and coercivity. Particularly, a rare earth magnet typified by a neodymium magnet (Nd2Fe14B) is used in various applications as an extremely strong permanent magnet because of its high magnetic flux density.
In a typical method for producing a permanent magnet, in order to obtain high magnetic flux density after sintering a raw material powder of the permanent magnet, crystal grains are rotated by intensive hot-working of a sintered body to form a texture composed of crystal grains oriented in the direction of an axis of easy magnetization (Patent Literature 1).
If the raw material powder has a structure composed of numerous coarse grains (typically, coarse crystal grains each having a crystal grain diameter of more than 300 nm) (coarse grain structure), coarse grains are less likely to rotate in the case of intensive work and thus the degree of orientation decreases, leading to reduction in residual magnetization. Coercivity also decreases due to coarse grains.
If the raw material powder has a structure composed of numerous amorphous, it is impossible to obtain an oriented structure that is made for a crystalline material to do, leading to a reduction in residual magnetization.
Accordingly, in order to ensure high degree of orientation by intensive hot-working to obtain large residual magnetization, it is important that the structure of the raw material powder is a nanocrystalline structure (typically having a crystal grain diameter of about 30 to 50 nm), which is neither a coarse grain structure nor an amorphous structure.
Therefore, there is a need to inspect the proportions of coarse grains or amorphous structures included in the raw material powder (coarse grain ratio or amorphous structure ratio).
In order to directly observe the structure of the raw material powder, a powder grain must be observed by TEM, SEM, or the like. However, it is difficult to apply the inspection of a coarse grain ratio or an amorphous structure ratio of the raw material powder by these methods of observing individual powder grains to actual industrial production.